Method for growing crystals from molten melts saturated with crystalline material

ABSTRACT

AN APPARATUS AND MMETHOD ARE DISCLOSED FOR GROWING CRYSTALS FROM A MELT OF A MOLTEN SOLVENT METAL THAT IS SATURATED WITH A SOLUTE. HEAT IS WITHDRAWN THROUGH A SEED CONTACTING THE SURFACE OF THE MELT TO PRECIPITATE THE SOLUTE ONTO SAID SEED AT THE SEED-MELT INTERFACE. THE SEED IS PULLED AWAY FROM THE MELT AT A RATE COMMENSURATE WITH THE PRECIPITATION RATES. SIMULTANEOUSLY, MORE SOLUTE IS DISSOLVED INTO THE MELT FROM A SOLUTE SOURCE IN THE MELT BENEATH THE GROWTH INERFACE WHILE MAINTAINING A CLOSE SOURCE-GROWTH INTERFACE SPACING SUBSTANTIALLY CONSTANT.

Apnl 24, 1973 c. E. BLElL 3,729,291

YSTALS FR METHOD FOR GROWING CR OM MOLTEN MELTS SATURATED WIT RYSTALLINEMATERIAL Original Filed Oct. 29, 1969 2 Sheets-Sheet 1 INVENTOR.

, Car! 5 624/ ZQQWM ATTORNEY Apnl 24, 1973 3,729,291

METHOD FOR GROWING CRYSTALS FROM MOLTEN MELTS SATURATED C. E. BLEIL WITHCRYSTALLINE MATERIAL Original Filed Oct. 29, 1969 2 Sheets-Sheet 2 Car/Z2 2? ATTORNEY United States Patent O1 3,729,291 Patented Apr. 24, 1973ice METHOD FOR GROWING CRYSTALS FROM MOLTEN MELTS SATURATED WITH CRYS-TALLINE MATERIAL Carl E. Bleil, Birmingham, Mich., assignor to GeneralMotors Corporation, Detroit, Mich.

Original application Oct. 29, 1969, Ser. No. 872,042. Divided and thisapplication Apr. 5, 1971, Ser. No. 131,278

Int. Cl. B015 17/04, 17/18 U.S. Cl. 23-301 SP 5 Claims ABSTRACT OF THEDISCLOSURE An apparatus and method are disclosed for growing crystalsfrom a melt of a molten solvent metal that is saturated with a solute.Heat is withdrawn through a seed contacting the surface of the melt toprecipitate the solute onto said seed at the seed-melt interface. Theseed is pulled away from the melt at a rate commensurate with theprecipitation rates. Simultaneously, more solute is dissolved into themelt from a solute source in the melt beneath the growth interface whilemaintaining a close source-growth interface spacing substantiallyconstant.

CROSS REFERENCES TO RELATED APPLICATION This is a division of Ser. No.872,042, filed Oct. 29, 1969, now U.S. Pat. No. 3,607,115. Thisapplication is also related to U.S. Ser. No. 795,561, Bleil, filed Jan.31, 1969, now U.S. Pat. No. 3,681,033.

BACKGROUND OF THE INVENTION Crystals have been grown in the past fromliquid melts of the crystalline materials. Crystals have also been grownfrom supersaturated aqueous solutions of the crystalline material. U.S.Pat. No. 3,031,275 Shockley describes a technique for growing crystalsfrom a melt floating on the surface of a liquid with which the melt isimmiscible. I have found a method and apparatus by which metal andsemimetal elemental and alloy crystals can be produced using many of thedesirable features of these prior techniques, without also incurring theancillary disadvantages of each.

Moreover, this invention can be used to produce continuous thin ribboncrystals. In my earlier U.S. patent application Ser. No. 795,561, Idescribe a novel technique for horizontal crystal growing which can beused to provide elemental thin crystalline ribbons of high purity. Bymodifying that technique in accordance with this invention, a variety ofadvantages can result. Ribbon growth temperature can be significantlyreduced. Compound and mixed crystal, i.e. alloy, ribbons of constantcomposition are readily formed. The length of the ribbon produced is notinherently limited by the starting volume of the melt.

SUMMARY OF THE INVENTION It is, therefore, a principal object of thisinvention to provide an improved method and apparatus for growingcrystals, particularly alloy crystals, from a saturated melt. A furtherobject of the invention is to provide an apparatus and method forproducing thin ribbons of alloy crystals. These and other objects of theinvention are attained by heating a solvent metal to a predeterminedtemperature at which it is molten, and saturating the solvent metal witha solute that will produce the crystal desired. A source of additionalsolute is located within the melt beneath the crystal growth interface.A close spacing is maintained between the source and the growthinterface as growth proceeds. A crystalline seed is placed in contactwith the melt above the source and heat withdrawn through the seed toprecipitate the solute onto the seed. Concurrently, the crystalline seedis moved away from the melt at a rate commensurate with the rate of thesolute precipitation. Simultaneously, additional solute is dissolvedinto the melt from the source beneath the growth interface to maintainmelt composition substantially constant in the growth area.

BRIEF DESCRIPTION 'OF THE DRAWING Other objects, features and advantagesof the invention will become more apparent from the followingdescription of preferred examples thereof and from the drawing, in whichFIG. 1 shows an elevational view in partial section of a horizontalcrystal growing apparatus that includes a solute source beneath thegrowth interface;

FIG. 2 shows an enlarged isometric view of the solute source shown inFIG. 1;

FIGS. 3, 3a and 3b show fragmentary isometric views of alternativesolute sources for the apparatus shown in FIG. 1;

FIG. 4 shows a horizontal crystal growing apparatus with a recirculatingsolute source;

FIG. 5 shows an enlarged plan view of the solute source shown in FIG. 4;and

FIG. 6 shows a sectional view along the line 6-6 of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As already indicated, thisinvention involves alloy crystals by precipitation of a solute onto acrystalline seed from a melt saturated with that solute. A source ofsolute is provided in the melt within a predetermined distance from thegrowth interface, with this spacing being maintained substantiallyconstant during crystal growth. Moreover, this invention alsocontemplates providing a surface geometry on the solute source that willaid in controlling the geometry of the growth interface. While thislatter concept may not be of appreciable significance for verticalcrystal growing, it is of special importance when horizontally growingcrystals in accordance with this invention. Shaping the source surfacecan permit one to produce crystalline ribbons of different surfacegeometries, as formed. It is in this latter connection that thisinvention has particular benefits in that it can provide thin, widemonocrystals flat on one side and of predetermined shape on the other.

It should also be noted that this technique can be used to producemonocrystals or polycrystalline material, as one desires. It can be usedto produce substantially elemental crystals, mixed crystals and crystalsof compounds. However, the composition of the crystals produced willalways be a function of the solid solubility of the melt solvent in thecrystal being formed. Hence, for ultimate purity in elemental crystalsthis technique would not be preferred. However, it provides a precisetech nique for producing elemental crystals containing selected amountsof other substances, for producing crystals of solute-solvent compoundsof selected composition, and the like. Since for all practical purposesthis technique produces crystals containing two or more elements Iprefer to refer to it as an alloy crystal growing technique. Thecrystal, of course, may not be a true alloy but a mixture. However, forconvenience of expression I shall refer to it as an alloy.

The actual composition of the crystal will thus depend on thesolute-solvent combination used, the temperature of the melt, etc.Consequently, the invention has a wide applicability from the productionof elemental and compound semiconductor monocrystals to the crystals ofa variety of metals.

Further, this invention can be used for vertical crystal growing, aswill become apparent. However, the maximum benefits of the invention aremore fully realized when it is used in combination with the horizontalcrystal growing apparatus I have described and claimed in my earlier US.patent application Ser. No. 795,561.

Reference is now made to FIG. 1 to illustrate that preferred use. Exceptfor the additional provision of the solute source, the apparatus shownin FIG. 1 is essentially the same and operated essentially the same asdescribed in my earlier filed patent application Ser. No. 795,561, whichis intended to be incorporated herein by reference. For this reason thesimilar aspects of apparatus and method shall only be briefly describedhere.

A rectangular crucible rests on a crucible support 12 within aconventional crystal growing enclosure (not shown) which provides asuitable environment for crystal growth. The crucible is surrounded by aradio frequency heater 14. The crucible is slightly overfilled with thedesired melt, so that the free surface of the melt extends slightlyabove the lip 16 of crucible 10.

One end of the lower surface of a crystalline seed 18 contacts thesurface of the melt while the other end is secured to a seed holder 20for pulling the seed horizontally away from the melt.

A melt of 95% indium-5% germanium can be used to grow germanium crystalscontaining small amounts of indium at a growth temperature ofapproximately 400 C. A melt of 75% silver-% germanium can be used toproduce germanium crystals containing small amounts of silver at agrowth temperature of 700 C. In such instance the seed 18 is ofgermanium in which the major faces of the seed are parallel and in the(111) plane. When horizontally pulled, the direction of pull is 211 Theupper surface of the end of the seed touching the melt is contacted by aheat sink 26, which is maintained at an appropriate temperature by meansof an encircling radio frequency type heater 28. Cooling coils surroundthe heat sink support.

A special heater 22 is recessed under the melt surface beneath the seedadjacent crucible lip 16 over which the seed is pulled. Heater 22 liesparallel the crucible lip, and transverse to the ribbon being formed, tosupply ribbon heat lost by radiation. A spring loaded roller 24 in thecrucible lip 16 supports the ribbon as it is pulled over the lip. Hence,ribbon growth is restricted to the end of the ribbon under the heatsink.

Heat sink 26 has an appendage 32 thereon for controlling temperature andmelt surface height at the end of the growing seed. Appendage 32 has aheater therein for more precise temperature adjustments adjacent thegrowth interface. A graphite bulb 34, having a heater therein is loweredinto the melt at a rate commensurate with the crystalline growth tomaintain the free melt surface at a constant height above the lip 16 ofthe crucible.

The solute source is a graphite or ceramic tube 36, the construction ofwhich is more clearly shown in FIG. 2. The source tube 36 liestransverse tothe ribbon being grown. It has a flat surface portion 38with a plurality of slots 39 which are positioned beneath the growingend of the seed beneath and parallel to the growth interface. The slotsprovide a permeable opening in the source under the ribbon growthregion. For best results, the flat surface portion 38 should bemaintained within about inch of the growth interface. Close spacing ofsurface 38 to the growth-interface is desirable to increase the rate ofcrystal growth and regulate growth interface geometry. Transport of thesolute from the source to the growth interface is diffusion controlled.Hence, the closer the spacing of the source to the growth interface, thefaster growth can be achieved. Analogously, the closer the sourcesurface geometry parallel the natural growth geometry, the easier thesystem is to control. The natural ribbon growth interface in the systemshown is not perfectly horizontal. Hence, the source surface 38 iscanted somewhat to be substantially parallel to it. However, in someinstances one may prefer a different relative orientation.

There is also a concurrent in-diifusion of the melt solvent into thesource through the slots 39 in the tube 36, While the source isdiffusing out. Hence, it is desirable that the volume of solute in tube36 be appreciable to avoid any significant solute concentration changesduring crystal growth that would adversely affect the rate of growth orthe length of the ribbon to be produced.

Of course, for most systems the source should at least be at the melttemperature and preferably slightly higher, to prevent the melt andsource from freezing. For this reason a source heater 40 is included intube 36. Tube 42 extending into the source tube 36 can be used tointroduce additional solute into tube 36 should it be desired. As can beseen, tube 36 bends at a right angle to extend up above the surface ofthe melt at a point removed from the growth interface. This not onlyincreases source volume but provides a ready access to the tubeinterior.

The source opening through which the solute diffuses can be formed in aplurality of ways, only some of which are shown. The openings should belarge enough to permit the solute to diffuse out but not so large as topermit droplets of solute to pass through. The type, geometry anddimensions of the source openings can thus be varied widely, and to someextent be a function of the solutesolvent system involved.

FIG. 2 shows a series of parallel, longitudinally oriented slots 39 inan integral flat portion of source tube 36, providing the sourceopenings. FIGS. 3, 3a and 3b show alternative forms of source aperturesWhich can be used. FIG. 3 shows a source tube 44 having a separate flatplate 46 secured over a longitudinal opening therein. The plate 46 has aseries of parallel, longitudinally oriented slots therein, analogous tothat shown in FIG. 2.

In FIG. 3a, source tube 50 also has a fiat plate 52 secured over to alongitudinal opening therein, with a plurality of slots 54 running thelength of the plate providing means for the solute to diffuse out of thetube. However, in addition, transverse grooves 58 and 60 are provided inplate 52 to alter the rate of diffusion of the solute out of tube 50 inthose areas where the grooves intersect. This difference in rate ofdiifusion will affect solute concentration in the adjacent areas ofcrystal growth, which in turn will aifect the rate of growth in therelated regions of the crystal being formed. Hence, by contouring plate52 one can contour the lower surface of the ribbon which is being grown.

'FIG. 3b shows that the source tube containing the source need not becylindrical but can be rectangular in cross section, as source tube 62shows. It also shows that a porous barrier need not be slots but can beof any porous body, such as a sintered metal structure 64. It might alsobe a screen. It should also be noted that the porous barrier need not beflat. In fact, in some instances it may be preferred that it be curvedboth radially and/or axially.

FIG. 4 shows a ribbon growth apparatus analogous to that shown inFIG. 1. However, the construction of the source in this embodiment ofthe invention provides means for continuously circulating a differentmeans and adding more solute to the source. FIGS. 5 and 6 show thesource in greater detail. The source is a substantially annular closedcontainer 68 having a flat upper surface. One portion 72 of this flatsurface has a plurality of parallel longitudinally oriented slotstherein, similar to those already described providing a porous partitionbetween the solvent and the solute. It is registered beneath the growthinterface in the manner already described. A heater 74 extends downthrough an opening 76 into the annulus and extends around and under theporous portion 72, lying beneath the growth interface. Opening 76 isalso large enough to accommodate impeller 78 which continuouslyrecirculates the solute in annulus 68 to maintain its temperature andconcentration uniform throughout. The annulus is positioned within themelt in support member 80. As the solute is consumed during ribbongrowth, or as solvent concentration in the annulus rises, additionalsolute can be added by means of feeder tube 82, when spring biased gate84 is opened.

The preferred temperature and rate at which crystals should be grownwill obviously be a function of the solvent system which is employed, aswell as the quality of crystals desired. Solvents for use in growinggermanium crystals include indium, gold, silver and copper. Bismuthcrystals can be grown with silver as a solvent. Silicon crystals can begrown in gold, silver and aluminum solvents. Antimony and telluriumcrystals can be grown with gold as a solvent. Bismuth can be used as asolvent to grow crystals of copper, indium and mercury. Tin crystals canbe grown with iron as a solvent. Such crystals will contain varyingminor amounts of the solvent metal.

In view of the foregoing, it can be appreciated that this invention isespecially useful for horizontally pulling thin, flat crystallineribbons. However, it should also be acknowledged that it can be used inthe more conventional vertical Czochralski crystal growing technique.Crystal boules of any suitable cross section can be convenientlyvertically pulled on a substantially continuous basis. In such instance,the crystal pulling means, itself, will function as a heat sink toconductively remove heat from the crystal. The cooler parts of thecrystal growing apparatus will also absorb heat radiated from thefreshly grown crystal. Hence, no special heat sink need be provided.

It is to be understood that although this invention has been describedin connection with certain specific examples thereof no limitation isintended thereby except as defined by the appended claims.

I claim:

1. The method of crystal growing which comprises heating a quantity of asolvent metal to a predetermined temperture at which it is liquid, saidquantity of solvent metal having an upper surface, saturating saidsolvent metal at said temperature with a solute of which a crystal isdesired to form a crystal growing melt, contacting the upper surface ofsaid melt with a solute crystalline seed that has an upper surface,withdrawing heat from said melt vertically by conduction through theupper surface of said seed to precipitate said solute onto a bottomsurface of said seed, horizontally pulling said seed across said meltupper surface to withdraw said seed from said melt, withdrawing saidseed from said melt at a rate commensurate with the rate at which saidsolute precipitates on the seed bottom surface, maintaining a source ofliquid solute within said melt for controlled introduction into saidmelt beneath said seed, and releasing said liquid, and concurrentlydissolving liquid solute into said melt from a source of liquid solutebeneath said seed where said solute precipitates at a rate commensuratewith solute precipitation onto said seed, to maintain melt com- 6position substantially constant as solute precipitation proceeds.

2. The method of crystal growing as defined in claim 1 wherein thesource of liquid solute is a horizontal tube having a substantially flatupper surface through which liquid solute is introduced into the melt,and the fiat surface is maintained substantially parallel to and withinabout /s inch of the surface of the melt.

3. The method as defined in claim 1 wherein both the solvent and thesolute are metals.

4. The method of crystal growing as defined in claim 1 wherein thesolvent is a melt and the solute is a semiconductor.

5. The method of growing a crystalline ribbon which comprises heating aquantity of a solvent metal to a predetermined temperature at which itis a liquid, said quantity of solvent metal having an upper surface,saturating said solvent metal at said temperature with a solute of whicha crystal is desired to form a crystal growing melt, immersing a sourceof solute within said melt, the source being a horizontal tube having asubstantially flat surface through which solute is introduced into themelt, maintaining the solute liquid within the solute source,maintaining the flat surface of the solute source within about inch ofthe surface of the melt, placing a generally fiat crystal seed on themelt upper surface over said flat surface on said source, said crystalseed having an upper surface, a lower surface, and two ends, maintainingone end of said upper surface substantially coplanar with its adjacentmelt surface, establishing a temperature gradient in the meltperpendicular to said seed surfaces so as to provide a generallyhorizontal solute precipitation region commensurate with the thicknessof the ribbon desired, selectively removing heat by conduction from theupper surface of said seed at said one end to progressively precipitatesolute on said seed lower surface thereon, suppressing net loss of theheat of fusion from the balance of the seed contacting the melt,horizontally pulling the other end of the seed across said melt uppersurface to withdraw said seed from the melt, and withdrawing the seedfrom the melt at a rate commensurate with said progressive soluteprecipitation on said seed lower surface to progressively grow a fiatcrystalline ribbon of solute from said melt while maintaining meltcomposition substantially constant.

References Cited UNITED STATES PATENTS 2,698,467 1/1955 Tarquinee23--301 2,898,249 8/ 1959 Jensen 23-301 2,907,715 10/1959 Cornelison23-273 2,937,216 5/1960 Fritts et al. 23-301 2,992,903 7/1961 Imber23301 3,031,275 4/1962 Shockley 23---273 3,423,189 1/1969 Pfann 23-3013,460,998 8/ 1969 Mullin et a1. 23-301 3,494,745 2/ 1970 Herczog et a1.23-301 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner

